Overweight and obesity are occurring at an alarming rate in children and adolescents in the United States and throughout the world. Among adolescents, aged 12-19 years, it is estimated that 34% are overweight and 18% are obese. The prevalence of being overweight as a high school student in Baltimore City is an alarming 26% higher than the national average. Many health problems are associated with overweight and obesity in the young, including increased risk of diabetes, stroke, coronary heart disease, hypertension, and premature mortality as an adult. Interventions to decrease overweight and obesity in this population have focused on increasing physical activity and modifying dietary habits. Behavioral counseling techniques and self-monitoring have been shown to be effective intervention tools to gradually decrease body mass index (BMI). The behavioral counseling techniques are designed to be a partnership between the counselor and participant or patient in that they allow the participant to identify goals, barriers, and create solutions to lead to healther lifestyle choices. New and innovative solutions to monitoring physical activity and dietary habits are needed, as many healthcare providers have reported a lack of time to assess these during healthcare visits. The proposed project is a randomized clinical trial that examines the use of a behaviorally-based weight loss Smartphone application on decreasing BMI in a group of overweight high school aged adolescents who attend the Harriet Lane Clinic at the Johns Hopkins Hospital. The Smartphone application is used for self-monitoring of dietary habits and physical activity and will be combined with a one time behavioral counseling session implemented by the applicant. This study is informed by Social Cognitive Theory, which focuses on strategies to increase self-efficacy for healthy behaviors to enhance health behavior outcomes. To achieve these aims, 60 overweight and obese adolescents will be recruited for participation. Following randomization, half of the participants will receive the Smartphone application intervention and half will receive usual care through their healthcare provider at the Harriet Lane Clinic for four months. Changes in BMI, self-efficacy for healthy eating and physical activity, healthy eating habits, and physical activity behaviors will be analyzed for differences between the two groups while controlling for other possible covariates. Satisfaction with the intervention and Smartphone technology will be assessed. Testing this type of technology is imperative as it could allow busy healthcare providers new ways to deliver efficient and effective interventions aimed at reducing overweight and obesity.